Percussion rotary drilling is known per se. A percussion rotary drill possesses a mechanism inducing an axial loading to the rotary drill bit. The mechanism provides impact forces on the material being drilled so as to break up the material which aids the rotary drill bit in cutting though the material.
Resonance enhanced rotary drilling is a special type of percussion rotary drilling in which there is an oscillator vibrating at high frequency so as to achieve resonance with the material being drilled. This results in an effective use of the mechanical energy at the rotary drill bit thus increasing drilling efficiency when compared to standard percussion rotary drilling.
U.S. Pat. No. 3,990,522 discloses a percussion rotary drill which uses a hydraulic hammer mounted in a rotary drill for drilling bolt holes. It is disclosed that an impacting cycle of variable stroke and frequency can be applied and adjusted to the natural frequency of the material being drilled to produce an amplification of the pressure exerted at the tip of the drill-bit. A servo-valve maintains percussion control, and in turn, is controlled by an operator through an electronic control module connected to the servo-valve by an electric conductor. The operator can selectively vary the percussion frequency from 0 to 2500 cycles per minute (i.e. 0 to 42 Hz) and selectively vary the stroke of the drill-bit from 0 to ⅛ inch (i.e. 0 to 3.175 mm) by controlling the flow of pressurized fluid to and from an actuator. It is described that by selecting a percussion stroke having a frequency that is equal to the natural or resonant frequency of the rock strata being drilled, the energy stored in the rock strata by the percussion forces will result in amplification of the pressure exerted at the tip of the drill-bit such that the solid material will collapse and dislodge and permit faster drill rates in the range 3 to 4 feet per minute.
There are several problems which have been identified with the aforementioned arrangement and which are discussed below.
High frequencies are not attainable using the apparatus of U.S. Pat. No. 3,990,522 which uses a relatively low frequency hydraulic periodic impactor, (described as an oscillator, but which does not oscillate in the strictly technical sense). Accordingly, although U.S. Pat. No. 3,990,522 discusses the possibility of resonance, it would appear that the low frequencies attainable by its impactor are insufficient to achieve resonance enhanced drilling through many hard materials.
Regardless of the frequency issue discussed above, resonance cannot easily be achieved and maintained in any case using the arrangement of U.S. Pat. No. 3,990,522, particularly if the drill passes through different materials having different resonance characteristics. This is because control of the percussive frequency and stroke in the arrangement of U.S. Pat. No. 3,990,522 is achieved manually by an operator. As such, it is difficult to control the apparatus to continuously adjust the frequency and stroke of percussion forces to maintain resonance as the drill passes through materials of differing type. This may not be such a major problem for drilling shallow bolt holes as described in U.S. Pat. No. 3,990,522. An operator can merely select a suitable frequency and stroke for the material in which a bolt hole is to be drilled and then operate the drill. However, the problem is exacerbated for deep-drilling through many different layers of rock. An operator located above a deep-drilled hole cannot see what type of rock is being drilled through and cannot readily achieve and maintain resonance as the drill passes from one rock type to another, particularly in regions where the rock type changes frequently.
Some of the aforementioned problems have been solved by the present inventor as described in WO 2007/141550. WO 2007/141550 describes a resonance enhanced rotary drill comprising an automated feedback and control mechanism which can continuously adjust the frequency and stroke of percussion forces to maintain resonance as a drill passes through rocks of differing type. The drill is provided with an adjustment means which is responsive to conditions of the material through which the drill is passing and a control means in a down-hole location which includes sensors for taking down-hole measurements of material characteristics whereby the apparatus is operable down-hole under closed loop real-time control.
US2006/0157280 suggests down-hole closed loop real-time control of an oscillator. It is described that sensors and a control unit can initially sweep a range of frequencies while monitoring a key drilling efficiency parameter such as rate of progression (ROP). An oscillation device can then be controlled to provide oscillations at an optimum frequency until the next frequency sweep is conducted. The pattern of the frequency sweep can be based on a one or more elements of the drilling operation such as a change in formation, a change in measured ROP, a predetermined time period or instruction from the surface. The detailed embodiment utilises an oscillation device which applies torsional oscillation to the rotary drill-bit and torsional resonance is referred to. However, it is further described that exemplary directions of oscillation applied to the drill-bit include oscillations across all degrees-of-freedom and are not utilised in order to initiate cracks in the material to be drilled. Rather, it is described that rotation of the drill-bit causes initial fractioning of the material to be drilled and then a momentary oscillation is applied in order to ensure that the rotary drill-bit remains in contact with the fracturing material. There does not appear to be any disclosure or suggestion of providing an oscillator which can import sufficiently high axial oscillatory loading to the drill-bit in order to initiate cracks in the material through which the rotary drill-bit is passing as is required in accordance with resonance enhanced drilling as described in WO 2007/141550.
Published international application WO2011/032874 by the present inventor looks to provide further improvements to the methods and apparatus described in the prior art. This application discloses a method for the controlling the frequency of the oscillator in the resonance enhanced rotary drill within a range defined by Us (the compressive strength of material being drilled). The method further involves controlling dynamic force (Fd) of the oscillator within a range defined by Us. The Us is measured using a sensor.
A drilling module is disclosed in published patent application WO2012/076401, of the present inventor. The application discloses an apparatus for use in resonance enhanced rotary drilling, which comprises a load-cell for measuring static and dynamic axial loading and a load-cell for measuring static and dynamic axial loading. Typically a load-cell is positioned above a vibration isolation unit and a load-cell is positioned between a vibration transmission unit and the drill-bit. The load-cells are connected to a controller in order to provide down-hole closed loop real time control of the oscillator. However, whilst this system of control can be effective in some instances, further refinement of the sensing system is required in more sophisticated control systems.
The solutions described in WO2007/141550, US2006/0157280, WO2011/032874 and WO2012/076401 focus particularly on the nature of the oscillations of the resonator. These are only a subset of the degrees of freedom involved, and the methods and systems do not take into account other important aspects of RED, especially those which have been more recently developed. For example, the sensors and systems for estimation of material characteristics are too simple, for example relying on imprecise measurement of compressive strength, and not capable of accounting for other material characteristics at all. Consequently, there is a desire to make further improvements to the methods and apparatus described therein. It is an aim of embodiments of the present invention to make such improvements in order to increase drilling efficiency, increase drilling speed and borehole stability and quality, while limiting wear and tear on the apparatus so as to increase the lifetime of the apparatus. It is a further aim to provide an apparatus, and in particular a drilling module, which can deliver more precisely controlled resonance enhanced drilling, particularly when drilling through rapidly changing rock types.